Microelectromechanical system (“MEMS”) resonators are small electromechanical structures that vibrate at high frequencies and are often used for timing references, signal filtering, mass sensing, biological sensing, motion sensing, and other applications. MEMS resonators are considered a common alternative to quartz timing devices. In general, quartz resonators have a high quality factor and piezoelectric coupling. High quality factor indicates a low rate of energy loss relative to the stored energy of the resonator, i.e., the oscillations die out more slowly. However, one limitation for quartz resonators is that they are difficult to design in smaller sizes.
Typically, MEMS resonators are made of silicon using lithography based manufacturing processes and wafer level processing techniques. Designers and manufacturers have found that pure silicon resonators often demonstrate very high quality factors comparable to quartz crystals, for example, as described in Non-patent document 1, identified below. However, bare silicon is not piezoelectric and pure silicon resonators have high motional impedance making them unsuitable to replace quartz resonators in many applications.
In order to lower the motional impedance of MEMS resonator, some designs have added piezoelectric material, such as a layer of thin film of aluminum nitride (AlN), as described in Non-patent document 2, for example, identified below. In a typical piezoelectric MEMS resonator, a thin film of molybdenum may be sputtered onto the silicon followed by a layer of AlN and an additional layer of molybdenum. After thin film deposition, the metal layers, the AlN layer and the silicon are etched to form the resonator shape. With the resulting design, the lower and upper layers of molybdenum serve as electrodes to excite and detect the mechanical vibrations of the resonator.
FIG. 1 illustrates a conventional film bulk acoustic resonator. As shown, the film bulk acoustic resonator 10 includes a pair of substrates layer 11 and 12 with an oscillation space 20, and a piezoelectric laminated structure 14 arranged to face the oscillation space 20. The piezoelectric laminated structure 14 includes a lower electrode 15, a piezoelectric thin film 16 and an upper electrode 17, which are arranged in sequence from the side close to the oscillation space 20. Moreover, a lower insulating layer 13 is formed in contact with the lower surface of the lower electrode 15, and an upper insulating layer 23 is formed in contact with the lower surface of the upper electrode 17.
One limitation with the design of the film bulk acoustic resonator 10 shown in FIG. 1 is that the addition of the piezoelectric film 16 and the metal layers 15 and 17 on breaks the symmetry of the film bulk acoustic resonator 10. In other words, the top of substrate layers 11 and 12 is dissimilar to the bottom of such layers.
The asymmetrical design causes vibrations in the thickness direction of the resonator that result in energy leakage out of the resonator. Typically, the piezoelectric MEMS resonators have quality factors that are about an order of magnitude lower than bare silicon resonators at the same frequency. This low quality factor increases the noise in oscillator applications and increases the motional impedance.
One design that attempts to overcome the low quality factor of piezoelectric MEMS resonators is to increase the size of the resonator by using a higher order overtone design, for example, as described in Patent document 1, identified below. While a higher order overtone design directly decreases the motional resistance, it also increases the size of the resonator. Moreover, since the manufacturing cost of the resonator is proportional to the size, the larger resonator size is not preferred. In addition, even for larger resonators, the low motional impedance is still not sufficient for low noise oscillator applications and a higher quality factor is required.    Non patent document 1: V. Kaajakari, T. Mattila, A. Oja, J. Kiihamäki, and H. Seppä, “Square-extensional mode single-crystal silicon micromechanical resonator for low phase noise oscillator applications”, IEEE Electron Device Letters, Vol. 25, No. 4, pp. 173-175, April 2004.    Non patent document 2: G. Piazza, P. J. Stephanou, A. P. Pisano, “Piezoelectric Aluminum Nitride Vibrating Contour-Mode MEMS Resonators”, Journal of MicroElectro Mechanical Systems, vol. 15, no. 6, pp. 1406-1418, December 2006.    Patent document 1: U.S. Pat. No. 7,924,119.